Also, it is interesting to note that, while there can be similarities, I believe most writers have triggers to begin writing that are unique to them. Mine were turning 30 and having given birth to my son, two major life changes that made me face up to the fact if I wanted to be a writer, I had better get on and write then!
I also love writing triggers in the form of opening lines, photos, closing lines etc that encourage you to write something around them. They can make you really work your imagination. The theme for competitions can also act as good triggers (and can be useful for writing practice even if you don't enter the contest. If you do and win it or are shortlisted, even better though!).
I find the ideal opening line writing trigger is one that can give you all kinds of possibilities to work with. For example, I would find the line "He refused to cry again" a lot more interesting to work with than "The starship crashed into a crater". I could write stories on both and have fun doing so but the first one you could set in any time, any world come to that. The second is an immediate sci-fi or fantasy story, which is fine, but you are getting your genre and setting in one go here. (That can be useful when I want triggers for my flash fiction though and that is when I will want a line that does a lot of work for me!).